STDDEV(expr)
返回expr表达式的标准差
MySQL > select stddev(scan_rows)
from log_statis
group by datetime;
+---------------------+
| stddev(`scan_rows`) |
+---------------------+
| 2.3736656687790934 |
+---------------------+
MySQL > select stddev_pop(scan_rows)
from log_statis
group by datetime;
+-------------------------+
| stddev_pop(`scan_rows`) |
+-------------------------+
| 2.3722760595994914 |
+-------------------------+
STDDEV,STDDEV_POP,POP